Supported catalyst, electrode using the supported catalyst and fuel cell including the electrode

ABSTRACT

Provided are a supported catalyst, an electrode including the same, and a fuel cell using the electrode. The supported catalyst includes a carbon-based catalyst support and metal catalyst particles having an average diameter of 3.5 to 5 nm and an amount of 80 to 90 parts by weight based on 100 parts by weight of the supported catalyst in a multi-layer structure adsorbed on a surface of the carbon-based catalyst support. In the supported catalyst of the present invention, as small metal catalyst particles with an average diameter of 3.5 to 5 nm are dispersed with high concentration, high dispersion, and the multi-layer structure, catalytic efficiency is increased. A fuel cell having improved energy density and fuel efficiency characteristics can be prepared using an electrode formed using the supported catalyst.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of Korean Patent Application No.2005-85590, filed on Sep. 14, 2005, in the Korean Intellectual PropertyOffice, the disclosure of which is incorporated herein in its entiretyby reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Aspects of the present invention relate to a supported catalyst and afuel cell using the same, and more particularly, to ahigh-concentration, high-dispersity carbon-supported catalyst withimproved catalytic efficiency manufactured using a polyol process, inwhich metal catalyst particles, such as platinum, are impregnated on asurface of a carbon-based catalyst support through a polyol process soas to form a mono-layer structure or multi-layer structure, and a fuelcell using the carbon-supported catalyst.

2. Description of the Related Art

Fuel cells are sources of clean energy and have the potential to replacefossil fuels, since they have a high power density and a highenergy-conversion efficiency. Fuel cells can be operated at an ambienttemperature and can be miniaturized and hermetically sealed. Thus, fuelcells can be used in a wide range of applications such as zero-emissionvehicles, household power generating systems, mobile telecommunicationsequipment, medical equipment, military equipment, space equipment, andportable electronic devices.

Proton exchange membrane fuel cells (PEMFCs) or direct methanol fuelcells (DMFCs) are power-generating systems that produce direct currentthrough an electrochemical reaction of methanol, water, and oxygen.These fuel cells include an anode and a cathode where liquid and gas aresupplied and have a structure in which a proton conductive membrane isinterposed between the anode and the cathode. A catalyst is contained inthe anode and the cathode. The catalyst in the anode decomposes hydrogenor methanol to form protons which pass through the proton conductivemembrane and react with oxygen in the presence of the catalyst in thecathode, as part of an overall process that generates electricity.

As described above, the catalyst is contained in the cathode and/oranode of the fuel cell to promote the electrochemical oxidation of fueland/or the electrochemical reduction of oxygen.

In PEMFCs, a catalyst with platinum particles dispersed in an amorphouscarbon support is used as the catalyst for the anode and the cathode. InDMFCs, PtRu is used in the anode, and platinum particles or a catalystthat has platinum particles dispersed in a carbon support is used in thecathode.

Methods of manufacturing a supported catalyst with platinum particles orplatinum-ruthenium particles dispersed in a carbon support are disclosedin U.S. Pat. Nos. 6,686,308 and 6,551,960.

According to the methods of manufacturing described in theabove-identified patents, if metal catalyst particles such as platinumparticles are dispersed in the carbon support, the platinum particlesmay be covered with carbon particles to such an extent that the weightof the metal catalyst particles increases by 80% or more. An increase inthe size of the metal catalyst particles leads to problems inimpregnation. Thus, catalytic activities and uses decrease, leading to adecline in the performance of unit cells.

SUMMARY OF THE INVENTION

Aspects of the present invention provide a high-concentration supportedcatalyst having improved catalytic activities and utilization efficiencyin which catalyst metal particles are highly dispersed on a carbon-basedcatalyst support so as to form a mono-layer and in which other catalystmetal particles are impregnated on the catalyst metal particlemono-layer so as to form multi-layers, and provide a method ofmanufacturing the high-concentration supported catalyst.

Aspects of the present invention also provide an electrode including thesupported catalyst described above and a fuel cell having improvedenergy density and fuel efficiency characteristics by using theelectrode described therein.

According to an aspect of the present invention, there is provided asupported catalyst including: a carbon-based catalyst support; and amulti-layer structure of metal catalyst particles having an averagediameter of 3.5 to 5 nm adsorbed on a surface of the carbon-basedcatalyst support. The amount of metal catalyst particles is 80 to 90parts by weight based on 100 parts by weight of the supported catalyst.

According to another aspect of the present invention, there is alsoprovided a supported catalyst including: a carbon-based catalystsupport; and a mono-layer structure of metal catalyst particles havingan average diameter of 2.5 to 3 nm on the carbon-based catalyst support.The amount of metal catalyst particles is 60 to 80 parts by weight basedon 100 parts by weight of the supported catalyst.

According to an aspect of the present invention, there is provided amethod of preparing the supported catalyst, the method including: (a)mixing a catalyst metal precursor and a polyalcohol to prepare a mixturecontaining the catalyst metal precursor; (b) mixing a carbon-basedcatalyst support and a mixture of a polyalcohol and water to prepare amixture containing the carbon-based catalyst support; and (c) mixing themixture containing the catalyst metal precursor and the mixturecontaining the carbon-based catalyst support, adjusting the pH of themixture, and heating the pH-adjusted mixture.

In the present invention, there may be a further method of preparing thesupported catalyst including: repeatedly performing operation (a)through operation (c) at least 1 additional time on the resultant fromoperation (c).

According to another aspect of the present invention, there is providedan electrode having the supported catalyst described above or formed bythe method described above.

According to another aspect of the present invention, there is provideda fuel cell having improved energy performance such as a cell potentialby using an electrode having the supported catalyst described above orformed by the method described above.

Additional aspects and/or advantages of the invention will be set forthin part in the description which follows and, in part, will be obviousfrom the description, or may be learned by practice of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

These and/or other aspects and advantages of the invention will becomeapparent and more readily appreciated from the following description ofthe embodiments, taken in conjunction with the accompanying drawings ofwhich:

FIGS. 1 and 2 are schematic diagrams of supported catalyst structuresaccording to embodiments of the present invention;

FIG. 3 is a flowchart illustrating a process of manufacturing asupported catalyst according to an embodiment of the present invention;

FIGS. 4A through 4C are graphs showing variables that influence the sizeof metal catalyst particles in the process of manufacturing thesupported catalyst according to an embodiment of the present invention;

FIG. 5 illustrates a structure of a fuel cell according to an embodimentof the present invention;

FIGS. 6 through 9 are Scanning Electron Microscopy (SEM) images ofsupported catalysts obtained according to Manufacturing Examples 1 and 4and Comparative Manufacturing Examples 1 and 5 of the present invention;and

FIGS. 10 and 11 are graphs showing a relationship between cell potentialand current density of the fuel cells prepared according to Examples 1and 2 and Comparative Examples 1 and 4 of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Reference will now be made in detail to the present embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings, wherein like reference numerals refer to the like elementsthroughout. The embodiments are described below in order to explain thepresent invention by referring to the figures.

FIGS. 1 and 2 are schematic diagrams of supported catalyst structuresaccording to embodiments of the present invention. A highly concentratedcarbon-supported catalyst according to an embodiment of the presentinvention has metal catalyst particles 11 in an amount of 60 to 80 partsby weight based on 100 parts by weight of the supported catalyst andhaving an average particle diameter of 2-3.5 nm. The metal catalystparticles 11 are combined on a carbon-based catalyst support 10 and mayform a mono-layer structure as illustrated in FIG. 1. Here, the term‘mono-layer’ refers to a layer dispersed on the carbon without anagglomeration of the metal catalyst particles 11 on the carbon-basedcatalyst support 10.

If the amount and the average diameter of the metal catalyst particles11 are below or above the ranges described above, catalytic activity maynot occur.

Referring to FIG. 2, a highly concentrated carbon-supported catalystaccording to another embodiment of the present invention has metalcatalyst particles 21 a and 21 b in an amount of 80 to 90 parts byweight based on 100 parts by weight of the supported catalyst and havingan average particle diameter of 3.5-5 nm. The metal catalyst particles21 a and 21 b form a multi-layer structure as illustrated in FIG. 2.Here, the term ‘multi-layer’ refers to a structure including acarbon-based support 20, first metal catalyst particles 21 a dispersedon the carbon, and second metal catalyst particles 21 b impregnated onthe first metal catalyst particles 21 a instead of impregnating on thesurface of the carbon-based support 20 where the first metal catalystparticles 21 a are not dispersed.

When the amount of the first and second metal catalyst particles 21 aand 21 b is below 80 parts by weight, the multi-layer structure may notbe formed and the use of the catalyst may decrease. On the other hand,when the amount of the first and second metal catalyst particles 21 aand 21 b is above 90 parts by weight, the size of the first and secondmetal catalyst particles 21 a and 21 b increases and catalytic activitymay not occur. Also, when the average diameter of the first and secondmetal catalyst particles 21 a and 21 b is below 3.5 nm or above 5 nm,catalytic activity may not occur.

In the supported catalyst according to the current embodiment of thepresent invention, the number of layers of the multi-layer structure ispreferably 2 or more for catalytic efficiency.

Hereinafter, referring to FIGS. 3 and 4A through 4C, a method ofmanufacturing the supported catalyst according to aspects of the presentinvention and variables in the process will be described below,according to embodiments of the present invention.

The manufacturing of the supported catalyst according to aspects of thepresent invention is performed by impregnating metal catalyst particlesinto a catalyst support. In this process which will be described below,a polyalcohol that acts as a reducing agent or a solvent is used. Themanufacturing process is referred to as a liquid catalyst impregnatingprocess.

A metal catalyst precursor is dissolved in a polyalcohol, and a mixturecontaining the metal catalyst precursor is prepared. As non-limitingexamples, the polyalcohol may be ethylene glycol, diethylene glycol ortriethylene glycol, and the amount of the metal catalyst precursor maybe 0.2 to 0.8 parts by weight, particularly 0.471 to 0.503 parts byweight, based on 100 parts by weight of the reaction solution (here, theweight of the reaction solution refers to a total of polyalcoholdissolving the metal precursor and polyalcohol dispersing carbon andwater, as described below). When the amount of the metal precursor isbelow 0.2 parts by weight, the total amount of the solution is increasedand the metal catalyst exists as colloid particles in the solutioninstead of forming on the carbon. When the amount of the metal precursoris above 0.8 parts by weight, the amount of the solution for the metalprecursor to reduce is insufficient and the size of the particles isgreatly increased.

The amount of the polyalcohol is 10⁵ to 2×10⁵ parts by weight based on100 parts by weight of the catalyst metal precursor. When the amount ofpolyalcohol is below 10⁵ parts by weight, the reduction ability ofprotons is decreased, thereby resulting in large catalyst particles.When the amount of polyalcohol is above 2×10⁵ parts by weight, thereduction ability of protons is increased, thereby resulting in manysmall catalyst particles, leading to agglomeration.

Examples of platinum precursors among the catalyst metal precursorsinclude tetrachloroplatinate (H₂PtCl₄), hexachloroplatinate (H₂PtCl₆),potassium tetrachloroplatinate (K₂PtCl₄), potassium hexachloroplatinate(K₂PtCl₆), and the like, or a mixture thereof. Ru precursors include(NH₄)₂[RuCl₆], (NH₄)₂[RuCl₅H₂O], and the like, and Au precursors includeH₂[AuCl₄], (NH₄)₂[AuCl₄], H[Au(NO₃)₄]H₂O, and the like.

In the case of an alloy catalyst, a mixture of precursors having amixing ratio corresponding to the desired atomic ratio of metals isused.

Separately, a mixture containing the carbon-based catalyst support isprepared by dispersing the carbon-based catalyst support with a mixtureof polyalcohol and water.

The carbon-based catalyst support is not particularly restricted, but isporous and has a surface area of 250 m²/g or more, preferably 500-1200m²/g and an average particle diameter of 10-500 nm, preferably 20-300nm. When the surface area is below this range, an impregnating abilityof the catalyst particles may be insufficient.

The carbon-based catalyst support satisfying the requirements describedabove may be at least one material selected from the group consisting ofcarbon black, Ketzen black (KB), acetylene black, activated carbonpowder, carbon molecular sieve, carbon nanotubes, activated carbonhaving micropores, and mesoporous carbon, as non-limiting examples. Inparticular, Ketzen black (KB) may be used.

In the reaction solution, that is, the mixture of polyalcohol and water,the amount of water is 40 to 60 parts by weight based on 100 parts byweight of the reaction solution. (here, the weight of the reactionsolution denotes a total of polyalcohol dissolving the metal precursorand the polyalcohol dispersing the carbon and water). When the amount ofwater is below this range, water as a reducing agent in a reductionprocess is insufficient and the size of the metal particles may beincreased. When the amount of water is above this range, theconcentration of polyalcohol is relatively low and the size of the metalparticles may be increased.

The carbon-based catalyst support may be hydrophilically modified, ifnecessary.

After the mixture containing the metal catalyst precursors is mixed withthe mixture containing the carbon-based catalyst support particlesaccording to the above-described process, the pH of the obtained mixtureis adjusted to a range of 9 to 13, such as, for example, a range of 10to 11, and then heated. When the pH of the mixture is below this range,the metal catalyst particles such as Pt particles may form a colloid andthus, a support may not be formed in the solution. When the pH of themixture is above this range, an agglomeration of Pt particles may beformed on the carbon and the size of the particles may be increased.

The heating temperature may be in a range of 90-115° C., such as, forexample, in a range of 105-110° C. The heating rate may be in a range of1.5 to 3.5° C./min, such as, for example, in a range of 2.1 to 2.4°C./min. When the heating temperature is below this range, a completereduction of the catalyst metal particles does not occur. When theheating temperature is above this range, a sudden boiling reaction ofthe reaction solution may occur and the amount of water in the reactionsolution may be inappropriate, increasing the size of the particles.When the heating rate is below this range, the generation speed of themetal catalyst particles such as Pt particles is slow and thus, the sizeof the particles increases. When the heating rate is above this range,the produced particles have a particle size that is too small and thusmay easily agglomerate.

After the heating process under the conditions described above, theresulting product is cooled to room temperature (25° C.) and then, thesupported catalyst of the present invention can be obtained using awork-up process including filtering, washing and lyophilization.

According to the method of manufacturing the supported catalystaccording to an aspect of the present invention, a supported catalysthaving a mono-layer structure of metal catalyst particles can beobtained, as illustrated in FIG. 1. In such a supported catalyst, theamount of the metal catalyst particles is 60 to 80 parts by weight basedon 100 parts by weight of the supported catalyst. The amount of thecarbon-based catalyst support is 20 to 40 parts by weight. The averagediameter of the metal catalyst particles is 2 to 3 nm.

In order to obtain a supported catalyst having a multi-layer structureof metal catalyst particles as illustrated in FIG. 2, the reductionprocess is repeatedly performed. In detail, the mixture of the metalcatalyst precursors obtained by dissolving the metal catalyst precursorsin polyalcohol and the mixture of the carbon-based catalyst supportobtained by dispersing the carbon-based catalyst support with themixture of polyalcohol and water are mixed with the product obtainedusing the heating process described above and then the heating processis repeatedly preformed, preferably 2 to 3 times. Here, the heatingtemperature and the heating rate are as described above.

After cooling the resultant obtained from the heating process to roomtemperature, the supported catalyst having the multi-layer structure canbe obtained using the work-up process including filtering, washing andlyophilization as in FIG. 2. In such a supported catalyst, the amount ofthe metal catalyst particles is 80 to 90 parts by weight based on 100parts by weight of the supported catalyst. The amount of thecarbon-based catalyst support is 10 to 20 parts by weight. The averagediameter of the metal catalyst particles is 3.5 to 5 nm.

In the supported catalyst obtained according to an embodiment of thepresent invention, the phenomenon that the metal catalyst particles aredispersed in the catalyst support having a mono-layer or multi-layerstructure may be identified through Transmission Electron Microscopy(TEM).

The supported catalyst prepared according to the above process may beapplied to an electrode catalyst layer of a fuel cell. In particular,the fuel cell may be a direct methanol fuel cell (DMFC).

Also, the supported catalyst according to aspects of the presentinvention may be used as a catalyst for various chemical reactionsincluding hydrogenation, dehydrogenation, coupling, oxidation,isomerization, decarboxylation, hydrocracking and alkylation.

FIG. 5 illustrates a structure of a DMFC according to an Example of thepresent invention.

Referring to FIG. 5, the DMFC includes an anode 32 where a fuel issupplied, a cathode 30 where an oxidant is supplied, and an electrolytemembrane 40 interposed between the anode 32 and the cathode 30. Ingeneral, the anode 32 includes an anode diffusion layer 22 and an anodecatalyst layer 33 and the cathode 30 includes a cathode diffusion layer32 and a cathode catalyst layer 31. In the current embodiment of thepresent invention, the anode catalyst layer 33 and the cathode catalystlayer 31 include the supported catalyst prepared according to thepreviously described process according to aspects of the presentinvention.

A bipolar plate 50 has a path for supplying the fuel to the anode 32 andacts as an electron conductor for transporting electrons produced in theanode 32 to an external circuit or an adjacent unit cell. A bipolarplate 50 has a path for supplying the oxidant to the cathode 30 and actsas an electron conductor for transporting electrons supplied from theexternal circuit or the adjacent unit cell to the cathode 30. In theDMFC according to an embodiment of the present invention, an aqueousmethanol solution is mainly used as the fuel supplied to the anode 32and air is mainly used as the oxidant supplied to the cathode 30.

The aqueous methanol solution transported to the anode catalyst layer 33through the anode diffusion layer 22 is decomposed into electrons,protons, carbon dioxide, and the like. The protons are transported tothe cathode catalyst layer 31 through the electrolyte membrane 40, theelectrons are transported to an external circuit, and the carbon dioxideis discharged to the outside. In the cathode catalyst layer 31, theprotons transported through the electrolyte membrane 40, the electronssupplied from an external circuit, and the oxygen in the air transportedthrough the cathode diffusion layer 32 react to produce water.

In such a DMFC, the electrolyte membrane 40 acts as a proton conductor,an electron insulator, a separator, and the like. The separator preventsunreacted fuel from being transported to the cathode 30 or unreactedoxidant from being transported to the anode 32.

In the DMFC according to the current embodiment of the presentinvention, materials for forming the electrolyte membrane 40 include acation exchanging polymer electrolyte, such as highly fluorinatedpolymer (for example, NAFION, available from Dupont), which issulfonated, having a main chain formed of fluorinated alkylene and aside chain formed of fluorinated vinyl ether having a sulfonic acidgroup on an end thereof.

Aspects of the present invention will be described in more detail withreference to the following examples. The following examples are forillustrative purposes and are not intended to limit the scope of thepresent invention.

Manufacturing Example 1

1.4656 g of the catalyst metal precursor, H₂PtCl₆.xH₂O (Umicore, Ptcontent: 39.8 wt %), was dissolved in 10 g of ethylene glycol (EG) andthen the mixture containing the catalyst metal precursor was prepared.0.25 g of the carbon (KB, S_(E)=800 m²/g) was added to 267 g of thesolution of EG and water (150 g of EG, 117 g of H₂O) and then, thesolution was dispersed by sonicating for 20 minutes. The metal precursormixture and the carbon-dispersed mixture were stirred for 10 minutes.Then the pH of the resultant was adjusted to 11 with a solution of 1MNaOH and once more, stirred for 10 minutes.

The resultant was maintained for 2 hours after raising the temperatureto 105° C. in 35 minutes (heating rate: 2.29° C./min) at roomtemperature (25° C.). The temperature of the resultant was then raisedto 110° C., maintained again for 1 hour, and then, cooled down to roomtemperature. The metal precursor solution (1.4656 g of H₂PtCl₆.xH₂O and10 g of EG) and 6 g of H₂O were added to the resultant at roomtemperature and the pH of the resultant was adjusted to 11. Such aheating process was performed once more. The obtained catalyst waswashed 3 to 4 times using a centrifugal separator and dried in alyophilizer. Finally, an 82.4 wt % Pt/C catalyst having a multi-layerstructure was prepared.

Manufacturing Example 2 Preparing Supported Catalyst

An 87.5 wt % Pt/C catalyst having a multi-layer structure was preparedin the same manner as in Manufacturing Example 1, except that thereduction process (that is, the operations of combining metal precursormixture and the carbon-dispersed mixture or resultant and heating) wasperformed once more (for a total of three repetitions).

Manufacturing Example 3 Preparing Supported Catalyst

An 85.7 wt % Pt/C catalyst having a multi-layer structure was preparedin the same manner as in Manufacturing Example 1, except that 1.8844 gof the catalyst metal precursor (H₂PtCl₆.xH₂O) and 367 g of the solutionmixed with EG and water (210 g of EG, 157 g of H₂O) were used.

Manufacturing Example 4 Preparing Supported Catalyst

A 90 wt % Pt/C catalyst was prepared in the same manner as inmanufacturing Example 3, except that a reduction process was performedonce more.

Manufacturing Example 5 Preparing Supported Catalyst

An 88.9 wt % Pt/C catalyst having a multi-layer structure was preparedin the same manner as in Example 1, except that 2.5126 g of the catalystmetal precursor (H₂PtCl₆.xH₂O) and 467 g of the solution mixed with EGand water (267 g of EG, 200 g of H₂O) were used.

Manufacturing Example 6 Preparing Supported Catalyst

A 92.3 wt % Pt/C catalyst was prepared in the same manner as inManufacturing Example 5, except that a reduction process was performedonce more.

Comparative Manufacturing Example 1 Conventional Catalyst

The product of Comparative Manufacturing Example 1 is a commerciallyobtainable catalyst, Pt-black (Johnson & Matthey, HiSPEC1000).

Comparative Manufacturing Example 2 Preparing Supported Catalyst

0.9422 g of the catalyst metal precursor, H₂PtCl₆.xH₂O (Umicore, Ptcontent: 39.8 wt %) was dissolved in 20 g of ethylene glycol (EG), andthen the mixture containing the catalyst metal precursor was prepared.0.25 g of the carbon (KB, S_(E)=800 m²/g) was added to 180 g of thesolution mixed with EG and water (80 g of EG, 100 g of H₂O) and then,the solution was dispersed by sonicating for 20 minutes. The metalprecursor mixture and the carbon-dispersed mixture were stirred for 10minutes. Then, the pH of the resultant was adjusted to 11 with asolution of 1M NaOH and once more, stirred for 10 minutes.

The resultant was maintained for 2 hours after raising the temperatureto 105° C. in 35 minutes (heating rate: 2.29° C./min) at roomtemperature (25° C.). The temperature of the resultant was then raisedto 110° C., maintained again for 1 hour, and then, cooled down to roomtemperature. The obtained catalyst was washed 3 to 4 times using acentrifugal separator and dried in a lyophilizer. Finally, a 60 wt %Pt/C catalyst having a mono-layer structure was prepared.

Comparative Manufacturing Example 3 Preparing Supported Catalyst

A 70 wt % Pt/C catalyst having a mono-layer structure was prepared inthe same manner as in Comparative Manufacturing Example 2, except that1.4656 g of the catalyst metal precursor (H₂PtCl₆.xH₂O) and 280 g of thesolution mixed with EG and water (160 g of EG, 120 g of H₂O) were used.

Comparative Manufacturing Example 4 Preparing Supported Catalyst

A 75 wt % Pt/C catalyst having a mono-layer structure was prepared inthe same manner as in Comparative Manufacturing Example 2, except that1.8844 g of the catalyst metal precursor (H₂PtCl₆.xH₂O) and 380 g of thesolution mixed with EG and water (220 g of EG, 160 g of H₂O) were used.

Comparative Manufacturing Example 5 Preparing Supported Catalyst

An 80 wt % Pt/C catalyst having a mono-layer structure was prepared inthe same manner as in Comparative Example 2, except that 2.5126 g of thecatalyst metal precursor (H₂PtCl₆.xH₂O) and 480 g of the solution mixedwith EG and water (280 g of EG, 200 g of H₂O) were used.

According to the above Manufacturing Examples 1 through 6 andComparative Manufacturing Examples 1 through 5, characteristics of thesupported catalyst were investigated and the results are shown in Table1.

TABLE 1 Number of support Theoretical ICP analysis with Number ofimpregnated XRD analysis TEM analysis Impregnated mono- reduction amountof Pt Diameter of Diameter of amount of Pt layer processes Parts by PtPt Parts by structure Time weight nm nm weight Comparative Pt-black 1time  100 7.7 12 100 Manufacturing Example 1 Comparative 60 1 time  602.65 2.93 56.9 Manufacturing Example 2 Comparative 70 1 time  70 2.823.02 67 Manufacturing Example 3 Comparative 75 1 time  75 3.02 2.89 70.7Manufacturing Example 4 Comparative 80 1 time  80 3.85 2.88 76.2Manufacturing Example 5 Manufacturing 70 2 times 82.4 3.85 3.37 77.3Example 1 Manufacturing 70 3 times 87.5 4.36 3.32 76.3 Example 2Manufacturing 75 2 times 85.7 4.18 3.74 82 Example 3 Manufacturing 75 3times 90 4.59 3.59 85.2 Example 4 Manufacturing 80 2 times 88.9 4.704.66 84.7 Example 5 Manufacturing 80 3 times 92.3 5.32 3.83 87 Example 6

For 60-80 wt % Pt/C catalysts in Table 1 that were manufactured througha single reduction process, it was identified through XRD and TEManalysis that small Pt particles having a diameter less than 3 nm wereformed on the carbon. When 2-3 continuous reduction processes wereperformed on such mono-layered support catalysts, catalysts having aparticle diameter 3.5-5 nm and an impregnated amount of 80-90 wt % couldbe prepared. For the other catalysts manufactured through continuousrepeated reduction processes, the amount of impregnated catalyst was ashigh as about 90 wt %, which increases the utilization efficiency of thesupported catalyst. In addition, the average particle diameters of thecatalysts was about 5 nm, which is smaller than the average particlediameter of 7.7 nm of the Pt-black catalyst. Thus, it is expected thatthe catalysts manufactured through repeated reduction processes havehigher catalyst activities.

In addition, TEM images of the supported catalysts of ManufacturingExamples 1 and 4 and Comparative Manufacturing Examples 1 and 5 wereobtained. The results are shown in FIGS. 6 through 9.

Referring to FIGS. 6 through 9, in the Pt-black catalyst of ComparativeManufacturing Example 1, it can be seen that the average particlediameter is as large as 12 nm, and thus the Pt particles are seriouslyagglomerated. In the Pt/C catalyst of Comparative Manufacturing Example5, Pt particles having a diameter less than 3 nm are uniformly dispersedon the carbon support so as to form a mono-layer. Compared with theseexamples, the Pt/C catalysts of Manufacturing Examples 1 and 4 have amulti-layered metal catalyst structure in which Pt particles aredispersed on the carbon support so as to form a mono-layer and in whichanother layer of Pt particles is formed on the mono-layer.

Example 1 Fabrication of a Fuel Cell

A fuel cell using a catalyst layer obtained using the supported catalystof Manufacturing Example 1 was fabricated as follows.

In forming an anode and a cathode used in the fuel cell of the presentExample, PtRu-black (HiSPEC 600) 5 mg/cm² and 3 mg/cm² (Pt standard) ofthe supported catalyst of Manufacturing Example 1 were sprayed onto thesurfaces of the anode and cathode diffusion layers 22 and 32. NAFION 115membrane was used as an electrolyte membrane. The obtained anode,cathode, and electrolyte membrane were joined under a pressure of 5 Mpaat 120° C. to prepare a membrane and electrode assembly (MEA). The MEAis a structure in which the catalyst layer and an electrode aresequentially laminated on both surfaces of a proton conductive polymermembrane.

A bipolar plate for supplying fuel and a bipolar plate for supplying anoxidant were attached to the anode and the cathode, respectively, of thefuel cell prepared according to Example 1, and then the performance ofthe fuel cell was determined. The operating conditions were as follows:0.28 mL/min of 1M aqueous methanol solution as fuel, 52.5 mL/min of airas an oxidant, and an operating temperature of 50° C.

Example 2 Fabrication of a Fuel Cell

A fuel cell was fabricated in the same manner as in Example 1, exceptthat the supported catalyst in Manufacturing Example 4 was used insteadof the supported catalyst in Manufacturing Example 1 during thefabrication of the catalyst layer.

Comparative Example 1 Fabrication of a Fuel Cell

A fuel cell was fabricated in the same manner as in Example 1, exceptthat the supported catalyst in Comparative Manufacturing Example 1 wasused instead of the supported catalyst in Manufacturing Example 1 duringthe fabrication of the catalyst layer.

Comparative Example 2 Fabrication of a Fuel Cell

A fuel cell was fabricated in the same manner as in Comparative Example1, except that the supported catalyst in Comparative ManufacturingExample 2 was used instead of the supported catalyst in ComparativeManufacturing Example 1 during the fabrication of the catalyst layer.

Comparative Example 3 Fabrication of a Fuel Cell

A fuel cell was fabricated in the same manner as in Comparative Example1, except that the supported catalyst in Comparative ManufacturingExample 3 was used instead of the supported catalyst in ComparativeManufacturing Example 1 during the fabrication of the catalyst layer.

Comparative Example 4 Fabrication of a Fuel Cell

A fuel cell was fabricated in the same manner as in Comparative Example1, except that the supported catalyst in Comparative ManufacturingExample 5 was used instead of the supported catalyst in ComparativeExample 1 during the fabrication of the catalyst layer.

In the fuel cells of Example 1 and Comparative Example 4, the change inthe cell potential with respect to current density was investigated andthe results are shown in FIG. 10. In the fuel cells of Example 2 andComparative Example 1, the change in the cell potential with respect tocurrent density was investigated and the results are shown in FIG. 11.

Referring to FIGS. 10 and 11, each of the fuel cells in Examples 1 and 2had a higher cell potential than that of Comparative Examples 1 and 4.

The current density of the fuel cells according to Examples 1 and 2 andComparative Examples 1 through 4 was investigated at 0.4 V at 50° C. andthe results are illustrated in Table 2.

TABLE 2 Current density (mA/cm²) Example 1 76.9 Example 2 85 ComparativeExample 1 61.1 Comparative Example 2 67.4 Comparative Example 3 64.1Comparative Example 4 63.1

While the fuel cell of Comparative Example 1 includes the Pt-blackcatalyst, the fuel cells of Comparative Examples 2 though 4 includesupported catalysts with a mono-layer structure having a particlediameter as small as less than 3 nm and an impregnated amount of 60-80wt %. However, referring to Table 2, the performances of the fuel cellsof Comparative Examples 2 though 4 having such a mono-layer structureare not so high for their small catalyst particle diameters. From theseresults, it was identified that, when Pt particles are embedded in thecarbon support and form a mono-layer structure, the utilizationefficiency of the catalyst does not increase even when the amount of theimpregnated Pt catalyst with respect to carbon is higher.

However, in the case of Examples 1 and 2, which have a multi-layerstructure, since Pt particles were formed on Pt particles, usage of thePt particles increases in the supported catalysts. Since the impregnatedamount was increased to 80-90 wt %, the thickness of the catalyst layercould be decreased, thereby improving catalytic activities.

In a supported catalyst according to an embodiment of the presentinvention, when small metal catalyst particles having an averagediameter of 3.5 to 5 nm are dispersed therein with high concentration,the catalytic efficiency is increased. A fuel cell having improvedenergy density and fuel efficiency characteristics can be prepared usingelectrodes formed using the supported catalyst according to an aspect ofthe present invention.

Although a few embodiments of the present invention have been shown anddescribed, it would be appreciated by those skilled in the art thatchanges may be made in this embodiment without departing from theprinciples and spirit of the invention, the scope of which is defined inthe claims and their equivalents.

1. A supported catalyst comprising: a carbon-based catalyst support; anda multi-layer structure of metal catalyst particles having an averagediameter of 3.5 to 5 nm on the carbon-based catalyst support and whereinan amount of metal catalyst particles is 80 to 90 parts by weight basedon 100 parts by weight of the supported catalyst.
 2. The supportedcatalyst of claim 1, wherein the metal catalyst particles are formed ofat least one material selected from the group consisting of platinum(Pt), ruthenium (Ru), palladium (Pd), rhodium (Rh), iridium (Ir), osmium(Os) and gold (Au).
 3. The supported catalyst of claim 1, wherein themulti-layer structure of the metal catalyst particles is a structure inwhich second metal catalyst particles are impregnated on an uppersurface of first metal catalyst particles disposed on the carbon-basedcatalyst support.
 4. The supported catalyst of claim 1, wherein a numberof layers in the multi-layer structure of the metal catalyst particlesis 2 or
 3. 5. The supported catalyst of claim 1, wherein thecarbon-based catalyst support has a surface area of 250 m²/g or greaterand an average particle diameter of 10 to 500 nm.
 6. The supportedcatalyst of claim 1, wherein the carbon-based catalyst support comprisesat least one material selected from the group consisting of carbonblack, Ketjen black, acetylene black, activated carbon powder, carbonmolecular sieve, carbon nanotubes, activated carbon having micropores,and mesoporous carbon.
 7. An electrode having the supported catalystaccording to claim
 1. 8. A fuel cell including an electrode having thesupported catalyst according to claim
 1. 9. A fuel cell comprising: acathode; an anode; and an electrolyte membrane interposed between thecathode and the anode, wherein at least one of the cathode and the anodecontains the supported catalyst of claim 1.